Nucleotide sequence analysis of the cat gene of Proteus mirabilis: comparison with the type I (Tn9) cat gene.

In Proteus mirabilis PM13 chloramphenicol resistance is mediated by the cat gene, a single copy of which is present in both resistant and sensitive isolates and which reverts at a high frequency. RNA measurements show an about 8.5-fold increase in cat-specific mRNA in cells expressing the resistance phenotype as compared with those which are sensitive to chloramphenicol. DNA sequence analysis has revealed a high degree of homology between the P. mirabilis cat gene and the type I cat variant (Tn9), 76% at the amino acid level and 73% when nucleotides in the coding sequence are compared. Sequence homology between the strain PM13 cat variant and Tn9 cat was not apparent however in the 5' and 3' flanking regions. Segments of near identity were seen when the upstream sequence of the cat of P. mirabilis was compared with the 5' regions of the Salmonella typhimurium flagellin genes H1 and H2, which are alternately expressed by a flip-flop control mechanism involving an invertible promoter and a trans-acting product.